Gastrin-releasing peptide: different forms, different functions

All forms of the neuropeptide gastrin-releasing peptide (GRP) are derived from the precursor proGRP1-125. Amidated GRP18-27, which together with amidated GRP1-27 was long thought to be the only biologically relevant product of the GRP gene, is involved in a multitude of physiological functions and acts as a mitogen, morphogen, and proangiogenic factor in certain cancers. Recently, GRP has been implicated in several psychiatric conditions, in the maintenance of circadian rhythm, in spinal transmission of the itch sensation, and in inflammation and wound repair. The actions of GRP are mediated by the GRP receptor. Over the last decade, nonamidated peptides derived from proGRP, such as the glycine-extended form GRP18-28 and recombinant and synthetic fragments from proGRP31-125, have been shown to be biologically active in a range of tissues and in cancer cell lines. While GRP18-28 acts via the GRP receptor, the identity of the receptor for proGRP31-125 and its fragments has not yet been established. Nonamidated fragments are also present in normal tissues and in various cancers. In fact, proGRP31-98 is the most sensitive serum biomarker in patients with small cell lung cancer and is a significant predictor of poor survival in patients with advanced prostate cancer.

Phosphorylation of cyclin-dependent kinase 2 peptides enhances metal binding

The cyclin-dependent kinase CDK2 is inactivated by phosphorylation on either of the two neighbouring residues Thr14 or Tyr15. The effect of phosphorylation on metal ion binding has been investigated with peptides incorporating residues 6-20 of CDK2. The stoichiometry of Ca(2+) binding increased from 1 in the un- and singly-phosphorylated peptides to 2 in the doubly phosphorylated peptide, without large changes in the affinity (75-250 microM). In contrast although binding of ferric ions to the un-phosphorylated peptide was not detected, both singly- and doubly-phosphorylated peptides bound two Fe(3+) ions. Binding of Ca(2+) or Zn(2+) ions to the doubly phosphorylated CDK2 peptide did not cause any change in absorbance, but increased the affinity of the peptide for Fe(3+) ions. These results demonstrate that double phosphorylation of CDK2 peptides increases the stoichiometry of metal ion binding, and hence may contribute to the previously observed regulation of CDK2 activity by metal ions.

Interrelationships between circulating gastrin and iron status in mice and humans

The observations that the peptide hormone gastrin interacts with transferrin in vitro and that circulating gastrin concentrations are increased in the iron-loading disorder hemochromatosis suggest a possible link between gastrin and iron homeostasis. This study tested the hypothesis that gastrin and iron status are interrelated by measurement of iron homeostasis in mice and humans with abnormal circulating gastrin concentrations. Intestinal iron absorption was determined by (59)Fe uptake following oral gavage, and concentrations of duodenal divalent metal transporter-1 (DMT-1) and hepatic hepcidin mRNAs were determined by quantitative real-time PCR in agastrinemic (GasKO), hypergastrinemic cholecystokinin 2 receptor-deficient (CCK2RKO), or wild-type mice. Iron status was measured by standard methods in the same mice and in hypergastrinemic humans with multiple endocrine neoplasia type 1 (MEN-1). Iron absorption was increased sixfold and DMT-1 mRNA concentration fourfold, and transferrin saturation was reduced 0.8-fold and hepcidin mRNA expression 0.5-fold in juvenile GasKO mice compared with age-matched wild-type mice. In mature mice, few differences were observed between the strains. Juvenile CCK2RKO mice were hypergastrinemic and had a 5.4-fold higher DMT-1 mRNA concentration than wild-type mice without any increase in iron absorption. In contrast to juvenile GasKO mice, juvenile CCK2RKO mice had a 1.5-fold greater transferrin saturation, which was reflected in a twofold increase in liver iron deposition at maturity compared with wild-type mice. The correlation between transferrin saturation and circulating gastrin concentration observed in mutant mice was also observed in human patients with MEN, in whom hypergastrinemia correlated positively (P = 0.004) with an increased transferrin saturation. Our data indicate that, in juvenile animals when iron demand is high, circulating gastrin concentrations may alter iron status by a CCK2R-independent mechanism.

Rho GTPases and p21-activated kinase in the regulation of proliferation and apoptosis by gastrins

Gastrins, including amidated gastrin (Gamide) and glycine-extended gastrin (Ggly), accelerate the growth of gastrointestinal cancer cells by stimulation of proliferation and inhibition of apoptosis. Gamide and Ggly activate different G proteins of the Rho family of small GTPases. For example, Gamide signals Rac/Cdc42 to activate p21-activated kinase 1 while Ggly signals Rho to activate Rho-activated kinase. p21-activated kinase 1 and Rho-activated kinase induce changes in phosphorylation or expression, respectively, of proteins of the Bcl-2 family, which then affect the caspase cascade with consequent inhibition of apoptosis. In addition, interaction of p21-activated kinase 1 with beta-catenin results in phosphorylation of beta-catenin, which enhances its translocation in to the nucleus, activation of TCF4-dependent transcription, and proliferation and migration. The central role of the beta-catenin pathway in carcinogenesis suggests that specific inhibitors of p21-activated kinase 1 may in the future provide novel therapies for gastrointestinal malignancies.

Gastrin is not required for liver regeneration

BACKGROUND

Although several growth factors are known to be essential for liver regeneration, the role of gastrin remains controversial.

METHODS

Liver regeneration was examined in wild-type (WT) and gastrin-deficient (gastrin KO) mice at days 2 and 10 after partial (40%) hepatectomy by measurement of liver weight. Hepatocyte proliferation and circulating gastrin concentrations were measured at the same time points by immunohistochemistry and radioimmunoassay, respectively.

RESULTS

There was no significant difference in the rate of liver regeneration between gastrin KO and WT mice. Hepatocyte proliferation in both groups was increased at day 2 but had returned to baseline values by day 10. At day 2, hepatocyte proliferation in the gastrin KO mice was significantly higher than in WT animals, whereas at day 10, proliferation was significantly greater in the WT mice. The circulating gastrin concentration in the WT mice was significantly lower at day 10 than in unoperated WT animals.

CONCLUSION

This study suggests that gastrin is not essential for liver regeneration after partial hepatectomy.

Beta-catenin/Tcf-4 inhibition after progastrin targeting reduces growth and drives differentiation of intestinal tumors

BACKGROUND & AIMS

Aberrant activation of the beta-catenin/Tcf-4 transcriptional complex represents an initiating event for colorectal carcinogenesis, shifting the balance from differentiation toward proliferation in colonic crypts. Here, we assessed whether endogenous progastrin, encoded by a target gene of this complex, was in turn able to regulate beta-catenin/Tcf-4 activity in adenomatous polyposis coli (APC)-mutated cells, and we analyzed the impact of topical progastrin depletion on intestinal tumor growth in vivo.

METHODS

Stable or transient RNA silencing of the GAST gene was induced in human tumor cells and in mice carrying a heterozygous Apc mutation (APCDelta14), which overexpress progastrin but not amidated or glycine-extended gastrin.

RESULTS

Depletion of endogenous progastrin production strongly decreased intestinal tumor growth in vivo through a marked inhibition of constitutive beta-catenin/Tcf-4 activity in tumor cells. This effect was mediated by the de novo expression of the inhibitor of beta-catenin and Tcf-4 (ICAT), resulting from a down-regulation of integrin-linked kinase in progastrin-depleted cells. Accordingly, ICAT down-regulation was correlated with progastrin overexpression and Tcf-4 target gene activation in human colorectal tumors, and ICAT repression was detected in the colon epithelium of tumor-prone, progastrin-overexpressing mice. In APCDelta14 mice, small interfering RNA-mediated progastrin depletion not only reduced intestinal tumor size and numbers, but also increased goblet cell lineage differentiation and cell apoptosis in the remaining adenomas.

CONCLUSIONS

Thus, depletion of endogenous progastrin inhibits the tumorigenicity of APC-mutated colorectal cancer cells in vivo by promoting ICAT expression, thereby counteracting Tcf-4 activity. Progastrin targeting strategies should provide an exciting prospect for the differentiation therapy of colorectal cancer.

Phylogenetic analysis of the sequences of gastrin-releasing peptide and its receptors: Biological implications

The many biological activities of the hormone gastrin-releasing peptide (GRP), including stimulation of acid secretion and of tumour growth, are mediated by the gastrin-releasing peptide receptor (GRP-R). Here sequence comparisons are utilised to investigate the likely bioactive regions of the 125 amino acid GRP precursor and of GRP-R. Comparison of the sequences of the GRP precursor from 21 species revealed homology not only in the GRP region between amino acids 1 and 30, but also in C-terminal regions from amino acids 43 to 97. This observation is consistent with recent reports that peptides derived from the C-terminal region are biologically active. Comparison of the GRP-R sequence with the related receptors NMB-R and BRS-3 revealed that the family could be distinguished from other G-protein coupled receptors by the presence of the motif GVSVFTLTALS at the cytoplasmic end of transmembrane helix 3. Comparison of the sequences of the GRP-R from 21 species revealed that the most highly conserved regions occurred in transmembrane helices 2, 3, 5, 6 and 7, and in the third intracellular loop. These results will be important in guiding future structure-function studies of the GRP precursor and of GRP receptors.

CCK receptors and cancer

Over the past 20 years there has been considerable interest in the role of CCK receptors in gastrointestinal cancer. Initial excitement over reports of the detection by PCR of CCK-1 and CCK-2 receptor mRNA in a wide range of gastrointestinal tumours has been tempered by the realisation that the presence of receptor binding sites is much more restricted. The current consensus is that expression of CCK-1 and -2 receptor proteins is common only in tumours of neural or neuroendocrine origin. A striking example of this general rule has been provided by the detection of CCK-2 receptors by receptor autoradiography in more than 90% of medullary thyroid carcinomas. Despite the absence of CCK receptors from many gastrointestinal adenocarcinomas, evidence from animal models and from tumour cell lines in vitro suggests that the CCK-2 receptor may contribute to the development of esophageal and gastric adenocarcinomas, and further experimental work in these areas is clearly warranted. Promising therapeutic approaches include the development of radiolabelled gastrin/CCK derivatives for use in tumour imaging, and the use of appropriate selective antagonists for treatment of those tumour subtypes likely to express CCK receptors.

Recombinant C-terminal fragments of the gastrin-releasing peptide precursor are bioactive

C-terminal fragments from the precursor for gastrin-releasing peptide (GRP) have been detected in several human tumour types. We have previously demonstrated that recombinant human proGRP42-98 is biologically active. To investigate the regions responsible, proGRP42-98 was cleaved with thrombin, and the fragments purified by HPLC. Both proGRP42-79 and proGRP80-98 stimulated proliferation of the human colorectal carcinoma cell line DLD-1, but neither peptide bound to the GRP receptor or bombesin receptor subtype 3. We conclude that two distinct regions of the proGRP C-terminus are biologically active, via a receptor distinct from the known GRP receptors. This discovery opens the way for the development of selective antagonists that may offer new therapies for proGRP-producing tumours.

C-terminal fragments of the gastrin-releasing peptide precursor stimulate cell proliferation via a novel receptor

There are many precedents for the production from a single precursor of multiple peptides, with independent receptors and different bioactivities. Gastrin-releasing peptide (GRP) is initially synthesized as amino acids 1-27 of a 125-residue precursor, proGRP, and is subsequently cleaved and amidated to form GRP18-27. We investigated the hypothesis that C-terminal proGRP peptides are also biologically active. Human proGRP18-125 was expressed in Escherichia coli as a glutathione S-transferase fusion protein. Recombinant proGRP18-125 stimulated proliferation and migration of the human colorectal carcinoma cell line DLD-1. The observations that an antagonist selective for the GRP receptor did not inhibit activity in either proliferation or migration assays and that the recombinant peptide did not bind to either the GRP receptor or orphan receptor BRS-3 indicated that neither activity was mediated by the known GRP receptors. Recombinant human proGRP31-125 and proGRP42-98 were also prepared and shown to stimulate proliferation of DLD-1 cells and the human prostate carcinoma cell line DU145. The synthetic peptides proGRP47-68 and [Tyr79]proGRP80-97 stimulated inositol phosphate production, MAPK kinase activity, and proliferation and migration of DLD-1 cells. Binding sites for both radioiodinated synthetic peptides were demonstrated on DLD-1 cells. Each peptide was able to compete with the other for binding, and a GRP receptor antagonist did not inhibit binding of either peptide. We conclude that peptides derived from the C terminus of proGRP are biologically active and that their activity is mediated by a receptor distinct from the two known GRP receptors.

Production, secretion, and biological activity of the C-terminal flanking peptide of human progastrin

BACKGROUND & AIMS

Processing of progastrin, the 80-amino acid precursor of the hormone gastrin, generates a variety of peptides with distinct distributions and biological activities. However, little is known regarding the expression, secretion, and biological activity of the 6-amino acid C-terminal flanking peptide (CTFP) of progastrin. The objectives were to determine the concentration of CTFP in normal subjects and patients with gastrointestinal diseases and to investigate the biological activity of CTFP.

METHODS

CTFP, gastrin-amide (Gamide), glycine-extended gastrin (Ggly), and progastrin were measured using region-specific radioimmunoassay (RIA) in antral extracts and resected colorectal cancers (CRC) and in plasma from normal subjects (fasting and meal stimulated) and from patients with CRC, multiple endocrine neoplasia type 1 (MEN-1), or pernicious anemia. The effect of CTFP on proliferation, migration, and activation of the mitogen-activated protein kinase (MAPK) pathway in several types of gastrointestinal cell lines was determined.

RESULTS

CTFP is by far the predominant progastrin-derived peptide found in the antrum (4-fold higher than Gamide), resected CRC, and circulation (60-fold higher than Gamide) and is released after meal stimulation. The hypergastrinemic patients (MEN-1, pernicious anemia) had elevated plasma Gamide but unaltered CTFP demonstrating differential secretion of these 2 progastrin-derived peptides. Finally, CTFP stimulated proliferation and migration and activated MAPK of cells in culture.

CONCLUSIONS

The high and regulated expression of CTFP in healthy and diseased subjects combined with the evidence for biological activity of CTFP demonstrates that CTFP is not an inactive metabolite of progastrin processing but is a bioactive peptide with potential roles in the normal and diseased gastrointestinal tract.

Circulating gastrin is increased in hemochromatosis

Gastric acid production is important in intestinal iron absorption. The peptide hormone gastrin exists in both amidated and non-amidated forms, which stimulate and potentiate gastric acid secretion, respectively. Since non-amidated gastrins require ferric ions for biological activity in vitro, this study investigated the connection between iron status and gastrin by measurement of circulating gastrin concentrations in mice and humans with hemochromatosis. Gastrin concentrations are increased in the plasma and gastric mucosa of Hfe(-/-) mice, and in the sera of humans with HFE-related hemochromatosis. The discovery of a relationship between iron status and circulating gastrin concentrations opens a new perspective on the mechanisms of iron homeostasis.

Gastrin-releasing peptide and cancer

Over the past 20 years, abundant evidence has been collected to suggest that gastrin-releasing peptide (GRP) and its receptors play an important role in the development of a variety of cancers. In fact, the detection of GRP and the GRP receptor in small cell lung carcinoma (SCLC), and the demonstration that anti-GRP antibodies inhibited proliferation in SCLC cell lines, established GRP as the prototypical autocrine growth factor. All forms of GRP are generated by processing of a 125-amino acid prohormone; recent studies indicate that C-terminal amidation of GRP18-27 is not essential for bioactivity, and that peptides derived from residues 31 to 125 of the prohormone are present in normal tissue and in tumors. GRP receptors can be divided into four classes, all of which belong to the 7 transmembrane domain family and bind GRP and/or GRP analogues with affinities in the nM range. Over-expression of GRP and its receptors has been demonstrated at both the mRNA and protein level in many types of tumors including lung, prostate, breast, stomach, pancreas and colon. GRP has also been shown to act as a potent mitogen for cancer cells of diverse origin both in vitro and in animal models of carcinogenesis. Other actions of GRP relevant to carcinogenesis include effects on morphogenesis, angiogenesis, cell migration and cell adhesion. Future prospects for the use of radiolabelled and cytotoxic GRP analogues and antagonists for cancer diagnosis and therapy appear promising.

Synthesis, expression and biological activity of the prohormone for gastrin releasing peptide (ProGRP)

Gastrin-releasing peptide (GRP) has a widespread distribution and multiple stimulating effects on endocrine and exocrine secretions and metabolism. The prohormone for GRP (ProGRP, 125 amino acids) is processed to the amidated, biologically active end products GRP(1-27) and GRP(18-27). Amidated forms of GRP are putative autocrine or paracrine growth factors in a number of cancers including colorectal cancer. However, the potential role and biological activity of proGRP has not been investigated. Using a newly developed antisera directed to the N terminus of human proGRP, proGRP immunoreactivity was detected in all of the endometrial, prostate, and colon cancer cell lines tested and in nine of 10 resected colorectal carcinomas. However, no amidated forms were detected, suggesting an attenuation of processing in tumors. Recombinant proGRP was expressed as a His-tag fusion protein and purified by metal affinity chromatography and HPLC. ProGRP stimulated proliferation of a colon cancer cell line and activated MAPK, but unlike GRP(18-27)amide had no effect on inositol phosphate production. ProGRP did not compete with iodinated bombesin in binding assays on Balb-3T3 cells transfected with the known GRP receptors, GRP-R or BRS-3. We conclude that proGRP is present in a number of cancer cell lines and in resected colorectal tumors and is biologically active. Our results suggest that antagonists to GRP precursors rather than the amidated end products should be developed as a treatment for colorectal and other cancers that express proGRP-derived peptides.

Mitogenic effects of both amidated and glycine-extended gastrin-releasing peptide in defunctioned and azoxymethane-treated rat colon in vivo

Although there is abundant evidence that gastrin-releasing peptide acts as a mitogen in various carcinoma cell lines, the effect of administration of gastrin-releasing peptide on the colorectal mucosa in vivo has not been reported. The aims of this study were to determine whether continuous infusion of gastrin-releasing peptide stimulated proliferation or accelerated carcinogenesis in the rat gastrointestinal tract and other organs. The possible requirement for C-terminal amidation for mitogenic activity in vivo was also investigated. Proliferation was measured in the colon by metaphase index and by immunostaining for the proliferation marker Ki-67, and in other tissues by immunostaining alone. Acceleration of colorectal carcinogenesis was assessed by counting aberrant crypt foci after treatment with the carcinogen azoxymethane. Defunctioning of the rectum reduced both the proliferative index and the crypt height of the rectal mucosa of untreated rats. Treatment with amidated or glycine-extended gastrin-releasing peptide for 4 weeks using implanted mini-osmotic pumps resulted in a two- to three-fold increase in proliferation, and an increase in crypt height, in the defunctioned rectal mucosa (p<0.001), with smaller but significant increases in the caecum and distal colon. No changes in proliferation were detected in lung, pancreas or gastric mucosa. The numbers of aberrant crypt foci in the mid-colon, distal colon and rectum following treatment with azoxymethane were also significantly increased by infusion with amidated or glycine-extended gastrin-releasing peptide. We conclude that administration of gastrin-releasing peptide to mature rats stimulates proliferation and accelerates carcinogenesis in the colorectal mucosa, and that C-terminal amidation is not essential for either effect. Gastrin-releasing peptides could thus potentially act as promoters of colorectal carcinogenesis.

Glycine-extended gastrin stimulates cell proliferation and migration through a Rho- and ROCK-dependent pathway, not a Rac/Cdc42-dependent pathway

Both amidated gastrin (Gamide) and glycine-extended gastrin (Ggly) stimulate gastrointestinal cell proliferation and migration. Binding of Gamide to the cholecystokinin-2 receptor activates small GTP-binding proteins of the Rho family (Rho, Rac, and Cdc42), and dominant-negative mutants of Rho or Cdc42 block Gamide-stimulated cell proliferation and survival. In comparison, little is known about the Ggly signaling transduction pathway leading to cell proliferation and migration. The present study examined the roles of the small G proteins Rho, Rac, and Cdc42 in Ggly-induced proliferation and migration of the mouse gastric epithelial cell line IMGE-5. Ggly stimulated the activation of Rho and its downstream effector protein ROCK. The activation of Rho and ROCK mediated Ggly-induced cell proliferation and migration as inhibition of Rho by C3, or ROCK by Y-27632, completely blocked these effects of Ggly. Ggly also stimulated tyrosine phosphorylation of focal adhesion kinase, and stimulation was reversed by addition of C3 and Y-27632. In contrast to the effects of Rho and ROCK, inhibition of the Rac or Cdc42 pathways by expression of dominant-negative mutants of Rac or Cdc42 did not affect Ggly-induced cell proliferation and migration. These results demonstrate that Ggly stimulates IMGE-5 cell proliferation and migration through a Rho/ROCK-dependent pathway but not via Rac- or Cdc42-dependent pathways.

Signaling the junctions in gut epithelium

This Perspective summarizes recent developments in our understanding of the signaling pathways involved in the regulation of epithelial cell adhesion in the gut. The role of phosphatidylinositol 3-kinase signaling in the modulation of adherens junctions, and the connections between tight junctions and nuclear transcription factors, are discussed. The effect of gastrins on adherens and tight junctions is presented as an example of the regulation of adhesion by growth factors. The consequences of dysregulation of adherens junctions and tight junctions for human pathology are also considered.

Stimulation of proliferation and migration of a colorectal cancer cell line by amidated and glycine-extended gastrin-releasing peptide via the same receptor

Although amidated forms of gastrin-releasing peptide (GRP) have been identified as autocrine growth factors in small cell lung cancer, their role in the development and progression of colorectal carcinoma is less clear. In addition, the biological activity of non-amidated gastrin-releasing peptide has not been investigated in colorectal carcinoma cells. We therefore investigated the effect of bombesin (a homologue of gastrin-releasing peptide) on proliferation, migration and inositol phosphate production in the human colorectal carcinoma cell line DLD-1, and determined the ability of gastrin-releasing peptide receptor antagonists to inhibit these effects. We also compared the biological activities of amidated and non-amidated GRP in the same assays. Treatment with either bombesin, or amidated or non-amidated GRP resulted in significant increase in proliferation, and in migration in a wound-healing assay. Both the mitogenic and migratory effects of amidated and non-amidated forms were inhibited by the GRP receptor antagonist [D-Phe(6), Leu-NHet(13), des-Met(14)]-bombesin(6-13). The presence of GRP receptor mRNA and GRP binding sites in three colorectal carcinoma cell lines was demonstrated by RT-PCR and by binding of radiolabelled bombesin, respectively. Transfection of DLD-1 cells with a dominant negative phosphatidylinositol 3-kinase did not affect bombesin-stimulated cell proliferation, but inhibited bombesin-stimulated cell migration. Bombesin and GRPgly activated phospholipase C, mitogen-activated protein kinase and focal adhesion kinase. We conclude that both amidated and non-amidated forms of gastrin-releasing peptide accelerate proliferation and migration of DLD-1 human colorectal carcinoma cells via the gastrin-releasing peptide receptor, but that phosphatidylinositol 3-kinase is only involved in the cell migration signalling pathway. Our results suggest a potential role for gastrin-releasing peptide receptor antagonists in the management of colorectal carcinoma.

PPARalpha agonists stimulate progastrin production in human colorectal carcinoma cells

The three subtypes of peroxisome proliferator activated-receptors (PPARalpha, delta and gamma) control the storage and metabolism of fatty acids. Treatment of rats with the PPARalpha ligand ciprofibrate increases serum gastrin concentrations, and several lines of evidence suggest that non-amidated gastrins act as growth factors for the colonic mucosa. The aim of the present study was to investigate the expression of PPARs and the effect of PPAR ligands on gastrin production and cell proliferation in human colorectal carcinoma (CRC) cell lines. mRNAs for all three PPAR subtypes were detected by PCR in all CRC cell lines tested. The concentrations of progastrin, but not of glycine-extended or amidated gastrin, measured by radioimmunoassay in LIM 1899 conditioned media and cell extracts were significantly increased by treatment with the PPARalpha ligand clofibrate. Similar increases in progastrin were seen following treatment with the PPARalpha ligands ciprofibrate and fenofibrate, but not with bezafibrate, gemfibrozil or Wy 14643. The PPARgamma agonist rosiglitazone had no significant effect on progastrin production. The PPARalpha ligand clofibrate also stimulated proliferation of the LIM 1899 cell line. We conclude that some PPARalpha ligands increase progastrin production by the human CRC cell line LIM 1899, and that clofibrate increases proliferation of LIM 1899 cells. These studies have revealed a relationship between PPARs and gastrin, two regulatory molecules implicated in the pathogenesis of CRC.

Properties of the complex between recombinant human progastrin and ferric ions

Binding of ferric ions to the hormone glycine-extended gastrin17 is essential for biological activity (Pannequin, J., et al. (2002). J. Biol. Chem. 277: 48602-48609). The aims of the current study were to determine the properties of the complex between recombinant human progastrin6-80 and ferric ions. The stoichiometry and affinity of ferric ion binding were determined by fluorescence spectroscopy. The selectivity of metal ion binding and the stability of the 59Fe(III) progastrin6-80 complex were determined by equilibrium dialysis. The stoichiometry of 2.5 +/- 0.1 moles Fe/mole progastrin, and the apparent dissociation constant of 2.2 +/- 0.1 microM, were similar to the values previously determined for glycine-extended gastrin17 at pH 4.0. Of the four trivalent and seven divalent metal ions tested, only ferrous and ferric ions bound to progastrin6-80. The ferric ion-progastrin complex was extremely stable, with a half-life of 117 +/- 8 days at pH 7.6 and 25 degrees C. We conclude that recombinant human progastrin6-80 selectively binds ferrous and ferric ions with high affinity in a stable 2:1 complex.

Identity and regulation of stored and secreted progastrin-derived peptides in sheep

Amidated and nonamidated progastrin-derived peptides have distinct biological activities that are mediated by a range of receptor subtypes. The objective was to determine the nature of the stored and secreted progastrin-derived peptides and to investigate whether progastrin release is regulated by gastric acidity. Using an antiserum directed to the C terminus of progastrin for identification and to monitor purification, C-terminal flanking peptides (CTFP) of progastrin (prog(76-83), prog(77-83), and prog(78-83) in approximately equivalent amounts) were isolated and identified from extracts of sheep antrum using ion exchange, HPLC, and mass spectrometry. Only trace amounts of full-length progastrin were present. Progastrin CTFP was the predominant progastrin-derived peptide in the antrum [progastrin CTFP/gastrin amide (Gamide) = 3]. Similarly, progastrin CTFP was the major circulating form in the antral (CTFP, 710 +/- 62 pmol/liter; Gamide, 211 +/- 35 pmol/liter) and jugular (CTFP, 308 +/- 16 pmol/liter; gastrin amide, 32 +/- 3 pmol/liter) veins. Alteration of gastric acidity in sheep by iv infusion of a H/K-adenosine triphosphatase inhibitor or somatostatin or by intragastric infusion of HCl demonstrated that the CTFP concentrations changed, although to a lesser extent than the changes in circulating gastrin amide. We conclude that the CTFP of progastrin is the major stored and circulating species of the gastrin gene, and that it is secreted in a regulated fashion rather than constitutively. Because full-length progastrin is bioactive, but is only a minor antral and secreted form, determination of the biological activity of the C-terminal flanking peptides will be important for a complete understanding of gastrin endocrinology.

Developmental expression and biological activity of gastrin-releasing peptide and its receptors in the kidney

Mammalian gastrin-releasing peptide (GRP) has a widespread distribution and multiple stimulating effects on metabolism, release of regulatory peptides, gastrointestinal and pancreatic secretions, and behavior. GRP is a potent mitogen for a number of tumor types, including colon and lung. Although GRP is known to stimulate the growth of renal tumors, little is known of its synthesis, distribution, and receptors in the developing and mature kidney. Both Northern blot analysis and RT-PCR revealed the presence of GRP mRNA in ovine kidney from midgestation through to adulthood. GRP mRNA was detected in rat kidney from embryonic day 19 to postnatal day 30 by RT-PCR. Sequence-specific radioimmunoassay demonstrated the presence of substantial amounts of fully processed amidated GRP in the ovine renal cortex and medulla. The mRNA for the major receptor subtype, GRP-R, was present in fetal and adult sheep and rat kidneys. The mRNA for the low-affinity GRP receptor, bombesin receptor subtype-3 (BRS-3), was only detected in the rat kidney. In the ovine kidney, immunohistochemistry localized GRP predominantly to the thick ascending limb of the loop of Henle. mRNAs for GRP, GRP-R, and BRS-3 were detected in the human embryonic kidney cell line HEK293, and radioimmunoassay of cell extracts and conditioned media revealed the presence of proGRP but not the amidated form. However, amidated GRP did stimulate the proliferation of these cells. These studies demonstrate that the developing and mature kidney may be previously unidentified sites of autocrine or paracrine action for GRP.

Biological Activity and Ferric Ion Binding of Fragments of Glycine-Extended Gastrin

Nonamidated gastrins such as progastrin and glycine-extended gastrin17 (Ggly) induce cell proliferation and migration in vitro and colonic mucosal proliferation in vivo. Our earlier NMR study defined the structure of Ggly and showed that ferric ions are essential to its biological activity, with the first binding to Glu7 and the second to Glu8 and Glu9 (Pannequin, J. et al. (2002) J. Biol. Chem. 277, 48602-48609). The aims of this study were to define the minimum biologically active fragment of Ggly and to determine whether ferric ions were also required for its activity. Cell-proliferation studies with Ggly fragments containing the five glutamate residues showed that the nonapeptide LE(5)AYG, the octapeptide LE(5)AY, and the heptapeptides E(5)AY and LE(5)A were fully active and that their activity was dependent on the presence of ferric ions. The activity of the hexapeptides LE(5) and E(5)A and the pentapeptide E(5) was reduced and independent of the presence of iron. The stoichiometry of ferric ion binding to LE(5)AYG, LE(5)AY, and E(5)AY, determined by absorption spectroscopy, was 2 mol/mol. NMR spectroscopy showed that the nonapeptide LE(5)AYG and shorter fragments had no defined structure and that the iron-binding sites differed from those in Ggly. We conclude that, in contrast to amidated gastrins where the C-terminal tetrapeptide is the minimum bioactive fragment, the shortest fully active fragments of Ggly are the heptapeptides LE(5)A and E(5)AY. These observations indicate that extensive proteolytic processing may not completely inactivate Ggly and that bioactive forms that are not detected by current radioimmunoassays may be present in tissues and/or plasma.

Gastrins, cholecystokinins and gastrointestinal cancer

The gastrointestinal peptide hormones gastrin and cholecystokinin (CCK) are well known for their ability to stimulate gastric acid secretion and pancreatic enzyme secretion, respectively. The suggestion that gastrin and CCK might also promote the development of cancers of the gastrointestinal tract has been controversial, but an increasing body of evidence now supports the view that the amidated and non-amidated forms of gastrin act as growth factors via different receptors in different regions of the gut. For example, animal experiments indicate that amidated gastrins are involved in cellular differentiation and repair in the gastric mucosa, and synergize with Helicobacter pylori infection in the development of gastric carcinoma. In contrast, non-amidated gastrins stimulate colonic mucosal growth, accelerate the early steps in colorectal carcinoma formation, and are elevated in the tumour and circulation of patients with colorectal cancer. Although human pancreatic carcinomas express CCK-1 and CCK-2 receptors, the role of gastrins and CCK in pancreatic carcinogenesis is yet to be established. Further investigation of the possible role of the CCK-2 receptor in gastric and pancreatic neoplasia, and of the hypothesis that gastrin precursors act as autocrine growth factors in colorectal carcinoma, is warranted. However, therapies aimed at the gastrins must be targeted to the relevant gastrin/gastrin receptor combination.